This invention relates to Magnetic Resonance Imaging (MRI) and more particularly to receive coils for MRI.
Optimal image quality in Magnetic Resonance Imaging is frequently obtained through the combined use of a large transmit coil and a smaller receive-only surface coil. The large transmit coil provides a relatively uniform excitation field whereas the smaller receive coil provides an optimal Signal-to-Noise Ratio (SNR) over the volume of interest. One important variation of this idea is the use of phased-array receive coils to provide the SNR of a smaller coil, but with the coverage of the entire array.
Recently, new techniques have been developed in which phased-array coils are used to provide partial spatial information for image reconstruction in order to reduce the scan time. These techniques known as SMASH (SiMultaneous Acquisition of Spatial Harmonics) and SENSE (Sensitivity Encoding) require that the location of the individual coils are known and characterized prior to use. Consequently, the placement of the coils is restricted to predetermined locations. Additionally, placement of the coils generally requires operator intervention subject to human error to register or locate the coils within the MRI system prior to image acquisition.
Surface coils and phased-array surface coils have several additional aspects that can be problematic in MR imaging. For example, surface coils have a very non-uniform sensitivity profile and images obtained with such coils typically have severe shading artifacts. This problem becomes increasingly severe as the coil becomes smaller.
Another aspect of surface coils is that they can be placed anywhere on the patient. For example, flexible phased array coils (also known as FLAP coils) can be wrapped around the patient before the patient is introduced into the magnet. While this aspect of surface coils is typically viewed as beneficial, if the coils are not properly placed with respect to both the patient""s anatomy and the imaging volume of the MRI system, aliasing of image data can occur. This aliasing occurs when a large transmit coil has a spatial coverage that encompasses a portion of the patient beyond the nominal region of gradient linearity.
Because the magnetic field gradient is created by a physical gradient coil, the gradient cannot have an infinite extent. Thus, at some distance from the center of the gradient coil along the axis of the applied field gradient, the gradient coil makes no contribution to the local magnetic field. Consequently, in some region between the electromagnetic end and the point of maximum field change within the gradient coil, the magnetic field created by the magnet is identical to the magnetic field at some point within the active volume of the gradient coil. Thus, aliasing of image information becomes possible. This form of aliasing (hereinafter referred to as xe2x80x9clarge field of view (FOV) aliasingxe2x80x9d) occurs in the intersection of the following regions:
a) Within the excitation volume of a large transmit (i.e. Body) coil;
b) Beyond the turn-around points of the magnetic field gradient;
c) Within the homogeneous field region of the magnet;
d) Within the sensitive volume of the receive coil; and,
e) Within a region having an active MR signal source.
An MRI system which takes full advantage of the benefits afforded surface coils and surface coil arrays, while overcoming the undesirable effects of coil placement restrictions, MR signal shading and large FOV signal aliasing is needed.
A self-localizing receive coil system for use with a Magnetic Resonance Imaging (MRI) system comprises at least one surface coil assembly for placement adjacent to a region of interest to be imaged and a plurality of tracking devices attached to the surface coil assembly for use in indicating location and orientation of the surface coil assembly during imaging.